1. Field of the Invention
The present invention relates to an automatic gain adjustment device and automatic gain adjustment method, and more particularly, is suitably applied to an optical disc device, for example.
2. Description of the Related Art
In optical disc devices of this kind, methods using a signal phase, signal amplitude or the like are employed in order to automatically carry out gain adjustment of a feedback control system which is used in a servo control circuit such as a focus servo or tracking servo.
For the automatic gain adjustment method using a signal phase, a phase comparison technique using a phase comparator in a servo loop has been proposed to realize the automatic gain adjustment (refer to, for example, Japanese Patent Laid-Open No. 11-24705 (page 4, FIG. 3))
FIG. 1 shows a concrete example of a gain adjustment circuit 1 which has been conventionally used. In the gain adjustment circuit 1, a disturbance signal S1 which is a sine wave and composed of a predetermined single frequency generated in a disturbance generator 2 is supplied to one input port of a phase comparator 3, and at the same time, supplied to a feedback control system 5 via an adder 4.
The feedback control system 5 is a closed circuit including a compensator 6, amplifier 7 and controlled object 8 (actuator drive system, etc.). After being phase-compensated by the compensator 6, the disturbance signal S1 to be supplied is amplified by an amplifier 7 to cause the object 8 to be controlled, and to output a signal S2 that is to return to the adder 4. In this manner, feedback control is carried out in the feedback control system 5.
The output signal S2 of the controlled object 8, which is an output of the feedback control system 5, is supplied to a band-pass filter (BPF) 9. The band-pass filter 9 extracts the same frequency band component as that of the disturbance signal S1 from the output signal S2 of the controlled object 8. The extracted frequency band component is then supplied to the phase comparator 3 via the other input port thereof. After comparing phases of the inputted signals S1 and S3, the phase comparator 3 supplies a gain setting unit 10 at the next stage with the comparison results. The gain setting unit 10 adjusts in real time a gain of the amplifier 7 in the feedback control system 5 based on the comparison results obtained from the phase comparator 3 such that a phase difference, which is the comparison result, falls within a certain range.
A gain of the feedback control system is defined by frequency transfer function in general. It is ideally desirable that a gain of the amplifier in the open loop be adjusted such that an actual gain becomes 0 dB at the frequency (hereinafter referred to as a crossover frequency) where an open loop gain of the feedback control system becomes 0 dB.
As shown in FIG. 1, in a case where the disturbance signal S1 is set at a crossover frequency fc and supplied from the disturbance generator 2 to the feedback control system 5, the open loop gain determined by the compensator 6, amplifier 7 and controlled object 8 in the feedback control system 5 is represented by Y/X, where signal amplitudes at point A, X and Y are A, X, and Y, respectively.
Assuming that the open loop gain Y/X is G, a closed loop gain determined by the adder 4, compensator 6, amplifier 7 and controlled object 8 in the feedback control system 5 is represented by Y/A, which can be represented by G/(1+G) by using G.
When the gain adjustment is carried out with the disturbance signal S1 that has been set at the crossover frequency fc output from the disturbance generator 2, since the open loop gain G and closed loop gain G/(1+G) differ from each other as described above, the closed loop gain G/(1+G) at the crossover frequency fc becomes 0 dB. On the other hand, the open loop gain G does not become 0 dB.
Accordingly, it is necessary that a signal having a crossover frequency f′c be output from the disturbance generator 2 in order for the open loop gain G to become 0 dB at the crossover frequency fc. This involves additional steps of calculating the crossover frequency f′c from an actual measurement and the like previously.
When the open loop gain G does not become 0 dB at the crossover frequency fc after completion of the gain adjustment, an additional gain adjustment needs to be carried out with the crossover frequency f′c changed. In this case, also a frequency band of the band-pass filter 9 must be changed.
Further, phases of the two signals S1 and S3 must be detected for comparison by the phase comparator 3. However, actual signals contain noises and the like, with the result that it has been difficult to accomplish accurate detection of the phases.